Quantum-Dot Focal Plane Array Has Two-Color Capability

Mid- (3 to 5 μm) and long-wave (8 to 12 μm) infrared focal plane arrays have found a place in numerous security and military applications that require night-vision capability. Single-color detectors have been common, but monolithic two-color devices offer several advantages, such as the ability to detect the absolute temperature map of a scene.

Two-color detectors have been demonstrated using mercury cadmium telluride (MCT) and quantum-well detectors. Now a group at the University of New Mexico in Albuquerque — in collaboration with researchers from its start-up company, Zia Laser Inc., the University of Texas at Austin and BAE Systems in Nashua, N.H. — has developed a two-color detector based on quantum dots.

The focal plane array made of self-assembled quantum dots acquired this thermal image of a hand at 300K in the background and a soldering iron in the foreground. Courtesy of Sanjay Krishna, University of New Mexico.One problem in developing MCT focal plane arrays has involved finding large-area homogeneous materials, a result of fundamental variations during crystal growth. Using quantum wells avoids this problem but encounters another: The detectors suffer from a lack of normal-incidence absorption. Nanoscale quantum-dot detectors have been promising because they can be grown on large GaAs wafers with excellent uniformity and offer normal-incidence operation.

In the new work, the research team used self-assembled quantum dots in the two-color, 320 × 256 infrared focal plane array. The approach offers several advantages over quantum-well IR photodetectors, such as lower dark current, normal incidence detection, higher responsivity and improved radiation hardness. The two-color detector uses a voltage-tunable InAs/InGaAs/GaAs dot-in-well structure in which the InAs quantum dots are placed in an InGaAs well, which is then placed in a GaAs matrix.

The investigators tested the detector by taking thermal images at a temperature of 80 K. They used various optical filters to demonstrate operation at 3 to 5 μm and at 8 to 12 μm. The results show promise for quantum-dot focal plane arrays in applications where low registration error is important, such as in the imaging of a quickly changing scene.

They plan to improve performance by working on strain-compensated designs, which will allow for more absorbing layers. They also plan to optimize the process and integration by using resonant cavities to develop detectors with higher operating temperatures.